1. Field of the Invention
The present invention relates to an image forming method in which toner images are formed by developing electrostatic latent images which are formed by electrophotography etc. and to a toner useful for the image forming method.
2. Discussion of the Background
As for electrophotography, various methods have been disclosed in, for example, U.S. Pat. No. 2,297,691, and Japanese Patent Publications Nos. 49-23910 and 43-24748.
Images are typically formed as follows:
(1) a photoconductor is entirely charged with a charging device; PA1 (2) the photoconductor is exposed to imagewise light to form an electrostatic latent image thereon; PA1 (3) the electrostatic latent image is developed with a dry toner to form a toner image on the photoconductor; PA1 (4) the toner image is transferred onto an image receiving material such as a paper sheet; and PA1 (5) the toner image transferred on the image receiving material is then fixed upon application of heat and/or pressure to prepare a hard copy. PA1 (1) an image having a wide area can be fixed at a time; PA1 (2) it is not necessarily needed that heat energy is always supplied to a fixing device; PA1 (3) fixing degree of toner images is hardly affected by the constituents and/or thickness of the receiving materials; and PA1 (4) resolution of fixed toner images is not deteriorated in the fixing process because the toner images do not contact a member such as heat rollers during the fixing process. PA1 forming a toner image on a receiving material with a toner including a colorant and a binder resin, wherein the toner has the following melt viscosity property: EQU 0.06&lt;.eta.120/.eta.100&lt;0.15 PA1 heating the toner image to fix the toner image on the receiving material.
Dry toners for use in such image forming methods typically include a binder resin and a colorant, and optionally include a charge controlling agent and a releasing agent. The properties requisite for these dry toners include good fixing ability, charging ability, fluidity, stability to environmental changes, mechanical strength, and ability to be pulverized. In color toners such as yellow, magenta, cyan and black toners for use in formation of full color images, gloss and transparency of fixed toner images are important properties in addition to these properties. Recently, a need for gloss of images are increasingly required even for monochrome toner images, or two or three color toner images as well as full color toner images.
In attempting to improve the gloss of fixed toner images, various methods have been disclosed. Japanese Laid-Open Patent Publications Nos. 7-120996 and 7-199583 have disclosed methods in which the glosses of four color toner images are uniformed by relatively decreasing the gloss of the black toner image. Japanese Laid-Open Patent Publication No. 8-314300 discloses a toner having a fixing property in which the gloss of the resultant toner images is controlled so as to fall in a predetermined range even when the fixing temperature changes in a predetermined range. Japanese Laid-Open Patent Publication No. 9-269695 discloses an image forming method in which the gloss of toner images is controlled by changing at least one of fixing conditions such as fixing speed, fixing pressure, fixing temperature and fixing position.
Toner image fixing methods are broadly classified into two methods, i.e., contact fixing methods and non-contact fixing methods. Typical contact fixing methods include heat roller fixing methods. Typical non-contact fixing methods include flash fixing methods and oven fixing methods (i.e., methods of fixing toner in a heated atmosphere).
The background fixing techniques mentioned above relate to heat roller fixing methods. When heat roller fixing methods are used, a releasing agent is typically used in the toner used. This is because by including a releasing agent in a toner an offset problem in that toner images adhere to a heat roller can be avoided. Suitable releasing agents useful for preventing the offset problem include waxes such as low molecular weight polypropylene and polyethylene, which are typically included in background toners, and silicone oils which are typically coated on the surface of a heat roller.
When a releasing agent is included in a toner, the transparency of the resultant toner decreases, resulting in deterioration of color reproduction of the resultant color images. In addition, these releasing agents tend to rise to the surface of the toner particles (so-called "a bleed-out phenomenon"), and therefore a film tends to be formed on toner-charge applying members such as blades and carriers, resulting in deterioration of these toner-charge applying members (so-called "a spent-developer problem").
In the methods in which a releasing agent is continuously coated on the surface of a heat roller, it is difficult to uniformly coat the releasing agent on the surface of the roller for a long time, resulting in changes of the gloss of the resultant toner images. In addition, the methods have a drawback in that a large size fixing unit is needed because a releasing agent has to be continuously supplied to the surface of a heat roller.
Specific examples of the non-contacting fixing methods include oven fixing methods in which hot air is applied to a receiving material having toner images thereon to fix the images, and flash fixing methods in which visible light or infrared light, which can be absorbed in toners, irradiates toner images to heat and fix the toner images. Among these non-contacting methods, oven fixing methods are preferable for fixing full color toner images.
FIG. 1 is a schematic view illustrating an oven fixing device. In FIG. 1, numerals 1-4 denote an image receiving material having toner images T thereon, a heat source, a cover and a feeding belt. The rotating direction of the feeding belt 4 is shown by a solid line arrow. The image receiving sheet is fed in a direction shown by a broken-line arrow to fix the toner image T.
The advantages of the oven fixing methods are as follows:
However, the oven fixing methods have a drawback in that fixed toner images tend to have low gloss because pressure is not applied during a heating operation in the fixing process.
In attempting to improve this drawback, Japanese Laid-Open Patent Publication No. 9-190013 discloses a toner which includes a specified epoxy resin as a binder resin and by which toner images having a high gloss can be obtained even when used in a non-contact fixing method. According to this Publication, full color toner images having good preservation properties and good clearness can be stably obtained.
Japanese Laid-Open Patent Publication 10-39539 discloses a toner which includes inorganic fine particles and which produces toner images having a uniform gloss because the toner has a specified melt viscosity and a specified BET surface area.
As mentioned above, these background methods intend to stably obtain toner images having a high gloss. However, gloss of toner images is a matter of individual's tastes. Therefore, when toner images are fixed so that the resultant toner images have a constant high gloss using one of these fixing methods, the toner image cannot necessarily satisfy all persons.
Because of these reasons, a need exists for an image forming method in which toner images having desired gloss can be easily obtained without changing toners.